1. Technical Field of the Invention
The present invention relates to an optical disc recording apparatus for recording information in a recording layer provided on one surface of the optical disc and forming an image in a coloring layer provided on the other surface off the disc.
2. Description of the Related Art
Hitherto, recordable optical discs, such as a CD-R (Compact Disc-Recordable) and a CD-RW (Compact Disc-Rewritable) have been extensively used for recording a large amount of information. One surface (recording face) of this type of optical disc is provided with a recording layer, and information is recorded by radiating a laser beam to the recording layer according to the information to be recorded.
Meanwhile, in recent years, there has been proposed a technology in which a coloring layer that changes its color in response to heat or light is integrally provided with an optical disc, the coloring layer being provided on a label face opposite from the recording face to draw images in order to indicate the contents recorded on the optical disc. The label face is set to face an optical pickup, and a laser beam is radiated by the optical pickup to cause the coloring layer to change its color so as to form a visible image.
Such an optical disc will be explained with reference to the accompanying drawings. FIG. 4 is a side sectional view showing the construction of the optical disc. As shown in the drawing, an optical disc 200 has a structure in which a protective layer 201, a recording layer 202, a reflective layer 203, a protective layer 204, a thermo sensitive layer 205 and a protective layer 206 are deposited in this order. Among these layers, the recording layer 202 is formed of a groove (pit) 202a and a land 202b. 
As shown in FIG. 6, the groove 202a observed from the recording face is spiraled clockwise from an inner circumference toward an outer circumference.
To record information on the optical disc 200, the recording face is set to oppose an object lens 114 of the optical pickup, as shown in FIG. 4, the optical disc 200 is turned counterclockwise as observed from the recording face, as shown in FIG. 6, tracking control is carried out to cause a laser beam B to follow along the groove 202a from an end point Gs on the inner circumference side, and the laser beam is radiated according to the information to be recorded, thereby recording the objective information. There are various types of tracking control, including one, for example, in which a laser beam is divided into a main beam and an auxiliary beam adjacent before or after the main beam in the radial direction, and the object lens 114 is swung to right or left such that both of return lights of the auxiliary beam coincide when a certain groove 202a is aligned with the center of the main beam. These tracking control methods are approximately the same in that the irradiation position of a laser beam is controlled so as to maintain the symmetry of the intensity distribution, including not only the return light in a certain groove 202a but also the return lights in the lands 202b located on both sides of the groove 202a. 
Furthermore when information is recorded, focusing control is also carried out to maintain a constant distance between the object lens 114 and a disc surface even when the optical disc 200 is rotated, the control being accomplished by vertically moving the object lens 114 so as to follow a fluctuated vertical movement taking place as the optical disc 200 is rotated. There are various types of such focusing control, including one, for example, in which an optical system is disposed such that spot image formation of the return light reflected back by the optical disc 200 changes according to the distance with respect to the disc surface, and the object lens 114 is operated so as to maintain a constant condition of the spot image formation. These control methods are approximately the same in that the object lens 114 is operated to maintain the constant condition of the return light of the laser beam.
Meanwhile, to form an image on the optical disc 200, the optical disc 200 is set with its label face opposing the object lens 114 of the optical pickup, the optical disc 200 is rotated, and the laser beam B is applied to the optical disc 200 to perform main scanning by the relative movement as the optical disc 200 is rotated. At the same time, the optical pickup is moved from an inner circumference toward an outer circumference to cause the laser beam B to perform sub scanning. During the scanning, the laser beam B having an intensity that is sufficiently high to change the color of the thermo sensitive layer 205 is applied on the basis of dots (pixel data) so as to form an objective image.
When the optical disc 200 is set with its label face opposing the optical pickup, the tracking control becomes difficult for the reason described below.
First, when the optical disc 200 is set with its label face opposing the optical pickup, the concavo-convex relationship between the groove 202a and the land 202b observed from the object lens 114 side is reversed from that 2in the case where the optical disc 200 is set with its recording face opposing the optical pickup. If, therefore, the tracking control is to be conducted, a laser beam will follow the land 202b. 
The material used for all the protective layers 201, 204 and 206 is polycarbonate having a refractive index of about 1.5. The protective layer 201 is considerably thicker than the protective layers 204 and 206. The recording layer 202 is at a point of about 1.2 mm as observed from the recording face, while it is at a point of only about 0.02 mm as observed from the label face.
The object lens 114 is designed so that it is focused (or a laser beam forms a spot having a predetermined diameter) on the reflective layer 203 (the recording layer 202) when it opposes the recording face to record information thereon. Hence, when the object lens 114 thus designed opposes the label face, the resulting detection range of its intensity distribution makes more extensive than the range applied when the object lens 114 is set to oppose the recording face. This will make it difficult to control the irradiation position of a laser beam to follow the land 202b. In addition, a laser beam is absorbed due to the coloration of the thermo sensitive layer 205, leading to temporarily reduced return light. This is another factor not expected to be encountered when the object lens 114 is set to oppose recording face, and contributes also to the difficulty of tracking control when the optical disc 200 is set with its label face opposing the optical pickup.
Thus, if the optical disc 200 is set with its label face opposing the optical pickup in order to form an image, normal tracking control cannot be expected. Rather, therefore, an image must be formed without using the tracking control.
However, in a state where the tracking control is disabled, if the optical disc 200 is, for example, eccentrically rotated around a point C2 slightly away from its central point C1, as shown in FIG. 7, then an irradiation trajectory Lp of a laser beam will be a circle with its center at the point C2. As a result, the circle intersects with the groove 202a having its center at the point C1 a plurality of times (five times in FIG. 7) for each rotation of the optical disc 200.
If a laser beam crosses over the groove 202a (or the land 202b), then the condition of the return light of the laser beam undesirably varies even when the distance to a disc surface remains constant. More specifically, the condition of the return light varies not only when the distance to the disc surface changes due to the rotation of the optical disc but also when the eccentric rotation causes the laser beam to cross over the groove 202a (or the land 202b). Furthermore, these two types of variations are both caused by the rotation of the optical disc 200, so that their frequency components are close to each other and relatively low.
Therefore, in the construction for controlling the focus of a laser beam so as to maintain a constant condition of return light, there is no discrimination between the variation attributable to a changed distance to a disc surface caused by the rotation of the optical disc 200 and the variation attributable to the laser beam crossing over the groove 202a or the like. This prevents normal focusing control. For instance, when an optical disc 200 that is ideally flat with no undulation is rotated, the distance between the optical disc 200 and the object lens 114 always remains constant; therefore, once a focus is fixed, then there should be no need to adjust the focus thereafter. If, however, a laser beam crosses over the groove 202a or the like due to eccentric rotation, then the condition of the return light changes. As a result, the focus is readjusted to cancel such a change, thus preventing the focusing control from being normally carried out.
Thus, if the focusing control feature fails to normally function, then the line width of the irradiation of a laser beam varies from one place to another, preventing uniformity from being maintained. This leads to deterioration in the quality of an image to be formed.